As is known, security threads that allow correct interpretation even by visual inspection, without necessarily resorting to particular instruments or devices, have already been used in the production of security documents, such as for example bank notes, certificates, passports and the like.
Among solutions of this kind, mention should be made of a security thread, disclosed in EP 0 319 157, which is substantially constituted by a strip of plastic material on which a layer of metallic material, generally aluminum, is deposited; areas are provided thereon in which graphic markings, text or the like are written in hollow form, since they are formed by metal-free regions located so as to maintain the metallic continuity of the thread.
The most widely used solution for-producing this thread is the use of systems for demetallizing aluminum vaporized in vacuum on a polyester film.
Over time, it has been found that this type of execution, which initially had a good degree of security, can be circumvented, since counterfeiters, by virtue of the increased availability of technology, have succeeded in providing very valid simulations by virtue of punch transfer methods or even by using laser beams for marking.
Another known solution that used this type of thread entailed insertion in the paper with the so-called windowing technique, disclosed for example in U.S. Pat. No. 4,186,943, which provided for the insertion of the strip in the paper with protrusion of the thread in a selected and controlled position so as to be able to see all the characteristics of the security thread in reflected light in the places where it lies outside the paper and the texts or graphic markings without metal in transmitted light in the places where the thread is inserted in the paper, thus achieving a further function for such thread.
In this case also, counterfeiters have been able to simulate the thread system by depositing similar materials, by using a punch and/or laser beam technique.
To increase the degree of security, solutions have already been devised in which additional security elements were applied to the thread with a holographic technique or the like, but in these solutions the text region provided in hollow form, or with partial or complete covering of the text, as disclosed for example in EP 0 330 733, produced an absence of holographic detection also on the character or graphic markings, so that the counterfeiter could still use punch or laser application.